Examples of conventionally known fluid concentration measuring devices include one described in Document 1. The measuring method and the measuring device described in Patent Literature 1 are employed for measuring the concentration of a treatment liquid as a fluid for washing treatment of semiconductor wafers. A plurality of measuring bodies are provided at intermediate positions of a treatment liquid supply pipe; light-transmissive portions having different light path lengths of light passing through the treatment liquid are provided inside the respective measuring bodies; light from a light source is supplied to one of the light-transmissive portions which has a light path length according to the properties of the treatment liquid; the light having passed through the treatment liquid at that light-transmissive portion is received by a light detector and the intensity of the light is measured; and the concentration of the treatment liquid is obtained from the light intensity based on the Beer-Lambert law.